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Network Working Group B. Aboba, Ed.
Request for Comment: 4924 E. Davies
Category: Informational Internet Architecture Board
July 2007
Reflections on Internet Transparency
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document provides a review of previous IAB statements on
Internet transparency, as well a discussion of new transparency
issues. Far from having lessened in relevance, technical
implications of intentionally or inadvertently impeding network
transparency play a critical role in the Internet's ability to
support innovation and global communication. This document provides
some specific illustrations of those potential impacts.
Table of Contents
1. Introduction ....................................................2
2. Additional Transparency Issues ..................................4
2.1. Application Restriction ....................................4
2.2. Quality of Service (QoS) ...................................6
2.3. Application Layer Gateways (ALGs) ..........................7
2.4. IPv6 Address Restrictions ..................................8
2.4.1. Allocation of IPv6 Addresses by Providers ...........8
2.4.2. IKEv2 ...............................................8
2.5. DNS Issues .................................................9
2.5.1. Unique Root .........................................9
2.5.2. Namespace Mangling ..................................9
2.6. Load Balancing and Redirection ............................10
3. Security Considerations ........................................11
4. References .....................................................11
4.1. Informative References ....................................11
Acknowledgments ...................................................13
Appendix A - IAB Members at the Time of Approval ..................14
1. Introduction
In the past, the IAB has published a number of documents relating to
Internet transparency and the end-to-end principle, and other IETF
documents have also touched on these issues as well. These documents
articulate the general principles on which the Internet architecture
is based, as well as the core values that the Internet community
seeks to protect going forward. This document reaffirms those
principles, describes the concept of "oblivious transport" as
developed in the DARPA NewArch project [NewArch], and addresses a
number of new transparency issues.
A network that does not filter or transform the data that it carries
may be said to be "transparent" or "oblivious" to the content of
packets. Networks that provide oblivious transport enable the
deployment of new services without requiring changes to the core. It
is this flexibility that is perhaps both the Internet's most
essential characteristic as well as one of the most important
contributors to its success.
"Architectural Principles of the Internet" [RFC1958], Section 2
describes the core tenets of the Internet architecture:
However, in very general terms, the community believes that the
goal is connectivity, the tool is the Internet Protocol, and the
intelligence is end to end rather than hidden in the network.
The current exponential growth of the network seems to show that
connectivity is its own reward, and is more valuable than any
individual application such as mail or the World-Wide Web. This
connectivity requires technical cooperation between service
providers, and flourishes in the increasingly liberal and
competitive commercial telecommunications environment.
"The Rise of the Middle and the Future of End-to-End: Reflections on
the Evolution of the Internet Architecture" [RFC3724], Section 4.1.1
describes some of the desirable consequences of this approach:
One desirable consequence of the end-to-end principle is
protection of innovation. Requiring modification in the network
in order to deploy new services is still typically more difficult
than modifying end nodes. The counterargument - that many end
nodes are now essentially closed boxes which are not updatable and
that most users don't want to update them anyway - does not apply
to all nodes and all users. Many end nodes are still user
configurable and a sizable percentage of users are "early
adopters," who are willing to put up with a certain amount of
technological grief in order to try out a new idea. And, even for
the closed boxes and uninvolved users, downloadable code that
abides by the end-to-end principle can provide fast service
innovation. Requiring someone with a new idea for a service to
convince a bunch of ISPs or corporate network administrators to
modify their networks is much more difficult than simply putting
up a Web page with some downloadable software implementing the
service.
Yet, even while the Internet has greatly expanded both in size and in
application diversity, the degree of transparency has diminished.
"Internet Transparency" [RFC2775] notes some of the causes for the
loss of Internet transparency and analyzes their impact. This
includes discussion of Network Address Translators (NATs), firewalls,
application level gateways (ALGs), relays, proxies, caches, split
Domain Name Service (DNS), load balancers, etc. [RFC2775] also
analyzes potential future directions that could lead to the
restoration of transparency. Section 6 summarizes the conclusions:
Although the pure IPv6 scenario is the cleanest and simplest, it
is not straightforward to reach it. The various scenarios without
use of IPv6 are all messy and ultimately seem to lead to dead ends
of one kind or another. Partial deployment of IPv6, which is a
required step on the road to full deployment, is also messy but
avoids the dead ends.
While full restoration of Internet transparency through the
deployment of IPv6 remains a goal, the Internet's growing role in
society, the increasing diversity of applications, and the continued
growth in security threats has altered the balance between
transparency and security, and the disparate goals of interested
parties make these tradeoffs inherently complex.
While transparency provides great flexibility, it also makes it
easier to deliver unwanted as well as wanted traffic. Unwanted
traffic is increasingly cited as a justification for limiting
transparency. If taken to its logical conclusion, this argument will
lead to the development of ever more complex transparency barriers to
counter increasingly sophisticated security threats. Transparency,
once lost, is hard to regain, so that such an approach, if
unsuccessful, would lead to an Internet that is both insecure and
lacking in transparency. The alternative is to develop increasingly
sophisticated host-based security mechanisms; while such an approach
may also fail to keep up with increasingly sophisticated security
threats, it is less likely to sacrifice transparency in the process.
Since many of the fundamental forces that have led to a reduction in
the transparency of the IPv4 Internet also may play a role in the
IPv6 Internet, the transparency of the IPv6 Internet is not pre-
ordained, but rather represents an ideal whose maintenance will
require significant ongoing effort.
As noted in [NewArch], the technical cooperation that once
characterized the development of the Internet has increasingly given
way to a tussle between the interests of subscribers, vendors,
providers, and society at large. Oblivious transport may be desired
by developers seeking to deploy new services; providers may desire to
block unwanted traffic in the core before it impacts subscribers;
vendors and providers may wish to enable delivery of "value added"
services in the network that enable them to differentiate their
offerings; subscribers may be sympathetic to either point of view,
depending on their interests; society at large may wish to block
"offensive" material and monitor traffic that shows malicious intent.
While there is no architectural "fix" that can restore oblivious
transport while satisfying the interests of all parties, it is
possible for providers to provide subscribers with information about
the nature of the services being provided. Subscribers need to be
aware of whether they are receiving oblivious transport, and if not,
how the service affects their traffic.
Since the publication of the previously cited IAB statements, new
technologies have been developed, and views on existing technology
have changed. In some cases, these new technologies impact oblivious
transport, and subscribers need to be aware of the implications for
their service.
2. Additional Transparency Issues
2.1. Application Restriction
Since one of the virtues of the Internet architecture is the ease
with which new applications can be deployed, practices that restrict
the ability to deploy new applications have the potential to reduce
innovation.
One such practice is filtering designed to block or restrict
application usage, implemented without customer consent. This
includes Internet, Transport, and Application layer filtering
designed to block or restrict traffic associated with one or more
applications.
While provider filtering may be useful to address security issues
such as attacks on provider infrastructure or denial of service
attacks, greater flexibility is provided by allowing filtering to be
determined by the customer. Typically, this would be implemented at
the edges, such as within provider access routers (e.g., outsourced
firewall services), customer premise equipment (e.g., access
firewalls), or on hosts (e.g., host firewalls). Deployment of
filtering at the edges provides customers with the flexibility to
choose which applications they wish to block or restrict, whereas
filtering in the core may not permit hosts to communicate, even when
the communication would conform to the appropriate use policies of
the administrative domains to which those hosts belong.
In practice, filtering intended to block or restrict application
usage is difficult to successfully implement without customer
consent, since over time developers will tend to re-engineer filtered
protocols so as to avoid the filters. Thus over time, filtering is
likely to result in interoperability issues or unnecessary
complexity. These costs come without the benefit of effective
filtering since many application protocols began to use HTTP as a
transport protocol after application developers observed that
firewalls allow HTTP traffic while dropping packets for unknown
protocols.
In addition to architectural concerns, filtering to block or restrict
application usage also raises issues of disclosure and end-user
consent. As pointed out in "Terminology for Describing Internet
Connectivity" [RFC4084], services advertised as providing "Internet
connectivity" differ considerably in their capabilities, leading to
confusion. The document defines terminology relating to Internet
connectivity, including "Web connectivity", "Client connectivity
only, without a public address", "Client only, public address",
"Firewalled Internet Connectivity", and "Full Internet Connectivity".
With respect to "Full Internet Connectivity" [RFC4084], Section 2
notes:
Filtering Web proxies, interception proxies, NAT, and other
provider-imposed restrictions on inbound or outbound ports and
traffic are incompatible with this type of service. Servers ...
are typically considered normal. The only compatible restrictions
are bandwidth limitations and prohibitions against network abuse
or illegal activities.
[RFC4084], Section 4 describes disclosure obligations that apply to
all forms of service limitation, whether applied on outbound or
inbound traffic:
More generally, the provider should identify any actions of the
service to block, restrict, or alter the destination of, the
outbound use (i.e., the use of services not supplied by the
provider or on the provider's network) of applications services.
In essence, [RFC4084] calls for providers to declare the ways in
which the service provided departs from oblivious transport. Since
the lack of oblivious transport within transit networks will also
affect transparency, this also applies to providers over whose
network the subscriber's traffic may travel.
2.2. Quality of Service (QoS)
While [RFC4084] notes that bandwidth limitations are compatible with
"Full Internet Connectivity", in some cases QoS restrictions may go
beyond simple average or peak bandwidth limitations. When used to
restrict the ability to deploy new applications, QoS mechanisms are
incompatible with "Full Internet Connectivity" as defined in
[RFC4084]. The disclosure and consent obligations referred to in
[RFC4084], Section 4 also apply to QoS mechanisms.
Deployment of QoS technology has potential implications for Internet
transparency, since the QoS experienced by a flow can make the
Internet more or less oblivious to that flow. While QoS support is
highly desirable in order for real-time services to coexist with
elastic services, it is not without impact on packet delivery.
Specifically, QoS classes such as "default" [RFC2474] or "lower
effort" [RFC3662] may experience higher random-loss rates than others
such as "assured forwarding" [RFC2597]. Conversely, bandwidth-
limited QoS classes such as "expedited forwarding" [RFC3246] may
experience systematic packet loss if they exceed their assigned
bandwidth. Other QoS mechanisms such as load balancing may have
side-effects such as re-ordering of packets, which may have a serious
impact on perceived performance.
QoS implementations that reduce the ability to deploy new
applications on the Internet are similar in effect to other
transparency barriers. Since arbitrary or severe bandwidth
limitations can make an application unusable, the introduction of
application-specific bandwidth limitations is equivalent to
application blocking or restriction from a user's standpoint.
Using QoS mechanisms to discriminate against traffic not matching a
set of services or addresses has a similar effect to deployment of a
highly restrictive firewall. Requiring an authenticated RSVP
reservation [RFC2747][RFC3182] for a flow to avoid severe packet loss
has a similar effect to deployment of authenticated firewall
traversal.
As with filtering, there may be valid uses for customer-imposed QoS
restrictions. For example, a customer may wish to limit the
bandwidth consumed by peer-to-peer file sharing services, so as to
limit the impact on mission-critical applications.
2.3. Application Layer Gateways (ALGs)
The IAB has devoted considerable attention to Network Address
Translation (NAT), so that there is little need to repeat that
discussion here. However, with the passage of time, it has become
apparent that there are problems inherent in the deployment of
Application Layer Gateways (ALGs) (frequently embedded within
firewalls and devices implementing NAT).
[RFC2775], Section 3.5 states:
If the full range of Internet applications is to be used, NATs
have to be coupled with application level gateways (ALGs) or
proxies. Furthermore, the ALG or proxy must be updated whenever a
new address-dependent application comes along. In practice, NAT
functionality is built into many firewall products, and all useful
NATs have associated ALGs, so it is difficult to disentangle their
various impacts.
With the passage of time and development of NAT traversal
technologies such as IKE NAT-T [RFC3947], Teredo [RFC4380], and STUN
[RFC3489], it has become apparent that ALGs represent an additional
barrier to transparency. In addition to posing barriers to the
deployment of new applications not yet supported by ALGs, ALGs may
create difficulties in the deployment of existing applications as
well as updated versions. For example, in the development of IKE
NAT-T, additional difficulties were presented by "IPsec Helper" ALGs
embedded within NATs.
It should be stressed that these difficulties are inherent in the
architecture of ALGs, rather than merely an artifact of poor
implementations. No matter how well an ALG is implemented, barriers
to transparency will emerge over time, so that the notion of a
"transparent ALG" is a contradiction in terms.
In particular, DNS ALGs present a host of issues, including
incompatibilities with DNSSEC that prevent deployment of a secure
naming infrastructure even if all the endpoints are upgraded. For
details, see "Reasons to Move the Network Address Translator -
Protocol Translator (NAT-PT) to Historic Status" [RFC4966], Section
3.
2.4. IPv6 Address Restrictions
[RFC2775], Section 5.1 states:
Note that it is a basic assumption of IPv6 that no artificial
constraints will be placed on the supply of addresses, given that
there are so many of them. Current practices by which some ISPs
strongly limit the number of IPv4 addresses per client will have
no reason to exist for IPv6.
Constraints on the supply of IPv6 addresses provide an incentive for
the deployment of NAT with IPv6. The introduction of NAT for IPv6
would represent a barrier to transparency, and therefore is to be
avoided if at all possible.
2.4.1. Allocation of IPv6 Addresses by Providers
In order to encourage deployments of IPv6 to provide oblivious
transport, it is important that IPv6 networks of all sizes be
supplied with a prefix sufficient to enable allocation of addresses
and sub-networks for all the hosts and links within their network.
Initial address allocation policy suggested allocating a /48 prefix
to "small" sites, which should handle typical requirements. Any
changes to allocation policy should take into account the
transparency reduction that will result from further restriction.
For example, provider provisioning of a single /64 without support
for prefix delegation or (worse still) a longer prefix (prohibited by
[RFC4291], Section 2.5.4 for non-000/3 unicast prefixes) would
represent a restriction on the availability of IPv6 addresses that
could represent a barrier to transparency.
2.4.2. IKEv2
Issues with IPv6 address assignment mechanisms in IKEv2 [RFC4306] are
described in [RFC4718]:
IKEv2 also defines configuration payloads for IPv6. However, they
are based on the corresponding IPv4 payloads, and do not fully
follow the "normal IPv6 way of doing things"... In particular,
IPv6 stateless autoconfiguration or router advertisement messages
are not used; neither is neighbor discovery.
IKEv2 provides for the assignment of a single IPv6 address, using the
INTERNAL_IP6_ADDRESS attribute. If this is the only attribute
supported for IPv6 address assignment, then only a single IPv6
address will be available. The INTERNAL_IP6_SUBNET attribute enables
the host to determine the sub-networks accessible directly through
the secure tunnel created; it could potentially be used to assign one
or more prefixes to the IKEv2 initiator that could be used for
address creation.
However, this does not enable the host to obtain prefixes that can be
delegated. The INTERNAL_IP6_DHCP attribute provides the address of a
DHCPv6 server, potentially enabling use of DHCPv6 prefix delegation
[RFC3633] to obtain additional prefixes. However, in order for
implementers to utilize these options in an interoperable way,
clarifications to the IKEv2 specification appear to be needed.
2.5. DNS Issues
2.5.1. Unique Root
In "IAB Technical Comment on the Unique DNS Root" [RFC2826], the
technical arguments for a unique root were presented.
One of the premises in [RFC2826] is that a common namespace and
common semantics applied to these names is needed for effective
communication between two parties. The document argues that this
principle can only be met when one unique root is being used and when
the domains are maintained by single owners or maintainers.
Because [RFC4084] targets only IP service terms and does not talk
about namespace issues, it does not refer to [RFC2826]. We stress
that the use of a unique root for the DNS namespace is essential for
proper IP service.
2.5.2. Namespace Mangling
Since the publication of [RFC2826], there have been reports of
providers implementing recursive nameservers and/or DNS forwarders
that replace answers that indicate that a name does not exist in the
DNS hierarchy with a name and an address record that hosts a Web
service that is supposed to be useful for end-users.
The effect of this modification is similar to placement of a wildcard
in top-level domains. Although wildcard labels in top-level domains
lead to problems that are described elsewhere (such as "The Role of
Wildcards in the Domain Name System" [RFC4592]), they do not strictly
violate the DNS protocol. This is not the case where modification of
answers takes place in the middle of the path between authoritative
servers and the stub resolvers that provide the answers to
applications.
[RFC2826] section 1.3 states:
Both the design and implementations of the DNS protocol are
heavily based on the assumption that there is a single owner or
maintainer for every domain, and that any set of resources records
associated with a domain is modified in a single-copy serializable
fashion.
In particular, the DNSSEC protocol described in "Protocol
Modifications for the DNS Security Extensions" [RFC4035] has been
designed to verify that DNS information has not been modified between
the moment they have been published on an authoritative server and
the moment the validation takes place. Since that verification can
take place at the application level, any modification by a recursive
forwarder or other intermediary will cause validation failures,
disabling the improved security that DNSSEC is intended to provide.
2.6. Load Balancing and Redirection
In order to provide information that is adapted to the locale from
which a request is made or to provide a speedier service, techniques
have been deployed that result in packets being redirected or taking
a different path depending on where the request originates. For
example, requests may be distributed among servers using "reverse
NAT" (which modifies the destination rather than the source address);
responses to DNS requests may be altered; HTTP "gets" may be re-
directed; or specific packets may be diverted onto overlay networks.
Provided that these services are well-implemented, they can provide
value; however, transparency reduction or service disruption can also
result:
[1] The use of "reverse NAT" to balance load among servers supporting
IPv6 would adversely affect the transparency of the IPv6
Internet.
[2] DNS re-direction is typically based on the source address of the
query, which may not provide information on the location of the
host originating the query. As a result, a host configured with
the address of a distant DNS server could find itself pointed to
a server near the DNS server, rather than a server near the host.
HTTP re-direction does not encounter this issue.
[3] If the packet filters that divert packets onto overlay networks
are misconfigured, this can lead to packets being misdirected
onto the overlay and delayed or lost if the far end cannot return
them to the global Internet.
[4] The use of anycast needs to be carefully thought out so that
service can be maintained in the face of routing changes.
3. Security Considerations
Several transparency issues discussed in this document (NATs,
transparent proxies, DNS namespace mangling) weaken existing end-to-
end security guarantees and interfere with the deployment of
protocols that would strengthen end-to-end security.
[RFC2775], Section 7 states:
The loss of transparency at the Intranet/Internet boundary may be
considered a security feature, since it provides a well defined
point at which to apply restrictions. This form of security is
subject to the "crunchy outside, soft inside" risk, whereby any
successful penetration of the boundary exposes the entire Intranet
to trivial attack. The lack of end-to-end security applied within
the Intranet also ignores insider threats.
Today, malware has evolved to increasingly take advantage of the
application-layer as a rich and financially attractive source of
security vulnerabilities, as well as a mechanism for penetration of
the Intranet/Internet boundary. This has lessened the security value
of existing transparency barriers and made it increasingly difficult
to prevent the propagation of malware without imposing restrictions
on application behavior. However, as with other approaches to
application restriction (see Section 2.1), these limitations are most
flexibly imposed at the edge.
4. References
4.1. Informative References
[NewArch] Clark, D. et al., "New Arch: Future Generation Internet
Architecture",
http://www.isi.edu/newarch/iDOCS/final.finalreport.pdf
[RFC1958] Carpenter, B., Ed., "Architectural Principles of the
Internet", RFC 1958, June 1996.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS Field)
in the IPv4 and IPv6 Headers", RFC 2474, December 1998.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597, June 1999.
[RFC2747] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
Authentication", RFC 2747, January 2000.
[RFC2775] Carpenter, B., "Internet Transparency", RFC 2775, February
2000.
[RFC2826] Internet Architecture Board, "IAB Technical Comment on the
Unique DNS Root", RFC 2826, May 2000.
[RFC3182] Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore, T.,
Herzog, S., and R. Hess, "Identity Representation for
RSVP", RFC 3182, October 2001.
[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
J., Courtney, W., Davari, S., Firoiu, V., and D. Stiliadis,
"An Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246,
March 2002.
[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
March 2003.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[RFC3662] Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort
Per-Domain Behavior (PDB) for Differentiated Services", RFC
3662, December 2003.
[RFC3724] Kempf, J., Ed., Austein, R., Ed., and IAB, "The Rise of the
Middle and the Future of End-to-End: Reflections on the
Evolution of the Internet Architecture", RFC 3724, March
2004.
[RFC3947] Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
"Negotiation of NAT-Traversal in the IKE", RFC 3947,
January 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC4084] Klensin, J., "Terminology for Describing Internet
Connectivity", BCP 104, RFC 4084, May 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380, February
2006.
[RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, July 2006.
[RFC4718] Eronen, P. and P. Hoffman, "IKEv2 Clarifications and
Implementation Guidelines", RFC 4718, October 2006.
[RFC4966] Aoun, C. and E. Davies, "Reasons to Move the Network
Address Translator - Protocol Translator (NAT-PT) to
Historic Status", RFC 4966, July 2007.
Acknowledgments
The authors would like to acknowledge Jari Arkko, Stephane
Bortzmeyer, Brian Carpenter, Spencer Dawkins, Stephen Kent, Carl
Malamud, Danny McPherson, Phil Roberts and Pekka Savola for
contributions to this document.
Appendix A - IAB Members at the Time of Approval
Bernard Aboba
Loa Andersson
Brian Carpenter
Leslie Daigle
Elwyn Davies
Kevin Fall
Olaf Kolkman
Kurtis Lindqvist
David Meyer
David Oran
Eric Rescorla
Dave Thaler
Lixia Zhang
Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: bernarda@microsoft.com
Phone: +1 425 706 6605
Fax: +1 425 936 7329
Elwyn B. Davies
Consultant
Soham, Cambs
UK
Phone: +44 7889 488 335
EMail: elwynd@dial.pipex.com
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